In a server or a file server which provides a file sharing service, a technology referred to as a snapshot for acquiring an image of a volume of a file system at a certain point of time for the purpose of use in backup or recovery of a file or a file system exists (e.g. the Patent Literature 1, the Patent Literature 2).
By regularly acquiring a snapshot and mounting it at an appropriate location, even if a failure occurs in a file or a file system, the data can be recovered relatively easily by referring to the snapshot acquired when the file or file system was normal.
As for a snapshot, a differential snapshot which retains only differential data of updated data blocks is common, instead of a copy of the entire data. An image of a virtual volume of the file system can be configured from the differential data and the original data by using a management table.
A snapshot technique is known which provides snapshots of a volume in a storage system where block data communication is provided as well as in the server where a file service is provided.
While the number of snapshots which can be acquired usually has an upper limit, there is a request recently for acquiring and retaining a larger number of snapshots over the upper limit due to the background that it is desired to retain these snapshots for a long period and to acquire the snapshots at a shorter interval.